A belt retractor commonly comprises a belt spool that is rotatably mounted in a frame and has at least one flange and a disk that can be non-rotatably blocked on the frame. Force limiters on belt retractors are usually based on the principle of plastic material deformation. If the belt force exceeds a predefined value, then a belt spool rotation in the unwinding direction is permitted against a defined resistance. In prior-art designs of belt retractors, the hollow belt spool is rigidly connected at one axial end thereof to a torsion bar that is arranged coaxially in the cavity of the belt spool, the torsion bar being non-rotatably coupled at the opposite end to a disk that is non-rotatably blocked by a blocking mechanism on the frame of the belt retractor in case of locking. The force level at which the limiting proceeds is determined by the torsional stiffness of the torsion bar. With such a force limiter, the course of the force level cannot be varied to any great extent across the angle of rotation of the belt spool.
In case of belt retractors which are used in the rear seat area where no airbags are provided, there additionally is the problem that the belt spool rotation during force limiting has to be restricted, so that the vehicle occupant to be restrained does not come into contact with the back rest of the front seat. Force limiters available up to now only allow an unwinding of belt webbing which at maximum corresponds to one revolution of the belt spool.
The invention provides a force limiter for belt retractors with which the course of the force level can be varied in a wide range across the angle of rotation of the belt spool, so that especially a degressive and/or a progressive characteristic curve is also possible. Further, in other embodiments of the invention there is realized an unwinding of the belt webbing in the order of several revolutions of the retractor, and there is used a force limiting stop as a function of the angle of rotation of the belt spool.